How can i slow down particles after baking
Other applications like houdini and maya, softimage (ICE) have a re-time cache function. Such thing doesn´t exist in Blender.
The closest thing that could emulate your effect is if your simulation is CACHED in open VDB and you export the sequence. Then create a second particle emmiter (on a new scene) and retime the flow of the particles first, then from the previously exported open VDB, using LOAD CACHE options your new (retimed) emmitter should work with the previous VDB cached simulation.